The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact inasmuch as the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated herein by this reference. These patents described in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is fragmented to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort.
Hot retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,432, includes establishing a combustion zone in the fragmented mass and introducing an oxygen-supplying gaseous combustion zone feed into the fragmented mass to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the combustion zone feed into the fragmented mass, the combustion zone is advanced through the fragmented mass.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone. This heats the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting", in the oil shale. The kerogen decomposes into gaseous and liquid products, including gaseous and liquid hydrocarbon products, and into a residual solid carbonaceous material.
The liquid hydrocarbon products, together with water produced in or added to the fragmented mass, are collected at the bottom of the fragmented mass. An off gas also is withdrawn from the bottom of the fragmented mass. The off gas contains combustion gas, including carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous combustion zone feed that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
The off gas produced during retorting can contain carbon monoxide and sulfur compounds such as hydrogen sulfide. Hydrogen sulfide and carbon monoxide are extremely toxic gases.
Additionally, since it is generally considered that retorting of oil shale commences in a retorting zone when oil shale is heated to at least about 900.degree. F., the combustion zone formed is substantially hotter than about 900.degree. F. and can have a temperature of up to about 1800.degree. F.
The gases produced are, therefore, very hot and for these reasons it is desirable to seal an access drift which is in fluid communication with the fragmented mass so that workers in adjacent underground workings are isolated from the off gas produced in the fragmented mass during the retorting operations.
U.S. Pat. Application Ser. No. 28,226, filed on April 9, 1979, by Kilburn, which is assigned to the assignee of the present invention and incorporated herein by this reference, discloses an insulated bulkhead which seals an access drift adjacent a fragmented permeable mass of formation particles in an in situ oil shale retort.
The bulkhead includes a steel plate for closing the cross-sectional area of the drift. The periphery of the bulkhead is anchored in concrete in a slot cut into the walls, roof, and floor of the drift. A layer of heat insulating material is applied to the face of the bulkhead adjacent the hot portion of the fragmented mass and a second layer of heat insulating material covers the walls, roof, and floor of the drift adjacent the insulated face of the bulkhead plate to minimize thermal degradation of formation surrounding the periphery of the bulkhead. Thermal degradation of formation surrounding a bulkhead caused by hot gases from the retort can result in spalling and sloughing of formation from the wall of the drift, thereby causing a bulkhead to lose its gas sealing capability.
It has been found that insulating a bulkhead and the formation surrounding the bulkhead is an expensive and time-consuming process.
It is, therefore, desirable to provide an improved gas seal which can be economically installed in an access drift and which can withstand high temperatures of retort gases without structural failure for at least the active life of the retort.